I am having trouble discerning if this encounter that I have planned is balanced. It is supposed to be the boss fight for the current chapter.

PC INFO: A troupe of 6-7 level six adventurers, heavy on Magic and Tanks, light on skill monkies.

INFO: Wetsig is the leader for a local bandit camp, being hired by the Warlock Trian to kidnap people for a handsome profit. Wetsig is a genius among trolls, but still only average by human standards. The Scrag knows his abilities and weaknesses and exploits his strengths. Wetsig is a troll fighter 1/ ranger1

ENCOUNTER: The PCs make it to the mud dome that is Wetsig's layer, there he sits in a man-made pond that is 120 feet in diameter and 30 feet down at its deepest. A wooden bridge connects the throne to the door. After a short dialogue Wetsig dives into the lake and attempts to take out the part of the bridge that is connected to the door (Since buoys keep the rest of the bridge afloat balance checks ensue). It then becomes a mostly underwater fight with the scrag Wetsig. Wetsig yields if dropped under half his HP in non-regeneratable damage (Fire or Acid). Otherwise he fights till incapacitation.

So is this encounter to hard, even for a boss fight? Would this be an average encounter for their party size, therefore not a strong boss fight? Is this encounter weak sauce?

EDIT

The encounters previous to this have been pretty light, so they will be at pretty much full resources
EDIT 2:
The party now is only four party members

Level 6 Druid
Level 6 Warmage
Level 6 Monk
Level 6 Barbarian

This is not what I expected the players to make, but they are now fairly well-rounded.
EDIT 3

The players themselves are also not very innovative, they are blasty characters who don't really think much about utility.
Also we are sticking MOSTLY to core with the exception of the Complete series.

Ah. Don't have books at hand as I am at work. But yes. It should be interesting for you to see how they solve the fight. It's good in my opinion to make boss battles 1 part Fight 1 part Puzzle. Similar to Zelda boss battles.

Depending on how crafty your partys spellcaster and skill monkey (I assume they have at least one rogue or ranger or something) and what books they have this could easily go in ways you don't expect. SO depending on your party I'd say easy-just right. But kudos to them if they manage to out think you here.

Does the party have access to water breathing spells or items? If they do, this fight will be much less challenging and more tedious, as they'll be needing to make swim checks every time they move or attack, and missile weapons will be rendered useless.

Does the party have access to any spells or items that allow for blindsight and similar abilities? If they do, this again will make the encounter far less challenging as they'll be able to locate him through the muck fairly easily.

Keeping that in mind, you should give the party a chance to keep the bridge from being disabled. Missile weapons become useless when firing into water, so make sure they either have ways to counteract that or ways that they can fight him toe-to-toe without making the encounter a series of skill checks with some fighting involved when they aren't completely immobilized.

This encounter sounds really weak, quite honestly...unless the entire party is equipped in full plate, then I retract my statement. Try battling a vampire squid in a 60 foot bowl of water with the bloody thing swirling around like a giant washing machine. THAT is a boss fight. If you peppered in some lackies I think it'd be a lot more challenging...or even add a few dominated sharks.

I would recommend lackeys come every now and then. What you could do is make the Troll use the surroundings to his advantage. Give him water breathing and make him jump down into the water so he can regenerate every 1d4+2 rounds.